1. Field of the Invention
This invention relates to the field of flow channels and to the field of forming microminiature structures in an etchable substrate material. In particular, it relates to the formation of grooves in an etchable substrate material, the grooves being capped to form a flow channel.
2. Prior art
As shown in cross-section in FIG. 1A, a flow channel can be made by etching a groove 103 (often V-shaped) in a substrate material 101 (typically silicon) and covering that groove with a flat plate 102 (typically glass or silicon). Often, a plurality of such grooves 103 are constructed in parallel to form a flow restrictor. A cross-sectional view of such a flow restrictor is shown in FIG. 1B and a top view is shown in FIG. 1C. The top view shows clearly the restriction of flow from the chamber 104 through the grooves 103.
A covered V-shaped groove forms a flow channel with a triangular cross-sectional shape. Flow through such a triangular channel is a function of the fourth power of the effective diameter of the groove. The effective diameter is approximately equal to the diameter of the largest circle that can be inscribed within the triangular cross-section of the channel. Since the flow is so sensitive to the magnitude of the effective diameter, small changes in the width 105 of the groove result in large differences in flow rate through the groove for a given pressure drop. Thus, it is important to be able to manufacture grooves so that the width of the groove has a small tolerance.
The shape of a V-groove etched in silicon is determined by the orientation of the crystal planes in the silicon. It might seem, then, that the magnitude of the width of the groove is set by the crystal planes, thus enabling easy repeatability in the manufacture of grooves of a given width and tolerance. In practice, however, the width of a V-groove is dependent on many parameters such as lithography and mask tolerances, orientation of the silicon material, undercutting of the etch mask, and anisotropy of the silicon etchants. The combination of these variables can account for errors of up to approximately 5% (between 1 and 5 micrometers depending on the size of the V-groove) in the width of a groove.
Medical infusion devices are a typical application for the flow restrictor channels described above. A commercially useful flow rate for such devices is about 0.5 ml/hr. This flow rate is produced by, for example, a flow restrictor channel with a width of 23 micrometers, a length of 200 micrometers, and a differential pressure of 7.5 psi. A change of 1 micrometer (4.3%) in groove width, as may easily result due to the factors discussed above, results in a flow change of about 17%, greater than can be tolerated in a commercial device. A variation of .+-.5% in flow rate is desirable for these devices. In order to stay within this flow rate tolerance, the width of the groove needs to be controlled to within .+-.1.2% (approximately .+-.0.28 micrometers for a groove width of 23 micrometers).
The ability to control the width of a groove to a maximum error of 0.28 micrometers is very difficult using current methods. In particular, it is difficult to purchase silicon wafers with a flat which is aligned with sufficient accuracy to the 110 crystal direction along which the V-grooves will be aligned. For a given etch time, wafers having flats which are misaligned by more than 0.2.degree. will yield significant increases in groove width during the anisotropic silicon etch as compared to wafers with perfectly aligned flats.
Thus, there is a need for the ability to form grooves in a substrate material with greater accuracy than is currently possible. In particular, it is desirable to be able to adjust the width of the groove after it has initially been formed by etching.